Transcript The Protists

The Protists and the Origins of
Eukaryotes
Chapter 20
Characteristics of
Protists
• Eukaryotes
• Primarily unicellular (some
colonial and multicellular
exist)
• Metabolically diverse
• Structurally complex
• Asexual reproduction usual;
sexual reproduction diverse
• Basically, catch-all
kingdom!!
• Only real characteristics in
common are:
– Eukaryotes
– Prefer watery environments
Protist Classification
• Phylogeny not well-established
• Protists do not represent a monophyletic group
• Some are more closely related to animals than to
other protists
• For convenience, protists grouped by:
– Mode of nutrition (plant-like, animal-like, fungal-like)
– Movement
• Protists are essentially the earliest eukaryotes and
1st steps towards true multicellular organisms
The Importance of Protists
• Origins of eukaryotic cell
• Origins of multicellularity
• Origins of sexual reproduction
• All of these advances are represented in various
protists
Figure 27.1 Major Eukaryote Groups in an Evolutionary Context (Part 1)
Figure 27.1 Major Eukaryote Groups in an Evolutionary Context (Part 2)
Origins of Eukaryotic Cell
•
Modern eukaryotic cell arose in
several steps
–
–
–
–
–
•
Flexible cell surface
Cytoskeleton
Nuclear envelope
Digestive vesicles
Endosymbiotic acquisition of certain
Cytoskeleton
– Extends from PM to nucleus
– Probably helped form nucleus
– Phagocytosis --> formation of
organelles
– Locomotion
– Increase in cell size
– Mitosis: allows for eventual meiotic
division needed for sexual
reproduction
– Flexible cell surface: can fold inward
to increase surface area
• Probably lead to endocytosis methods
and vesicle formation
Evidence: Giardia
• Have nucleus
• Lack
mitochondria
• No other
membraneenclosed
organelles
• Has
cytoskeleton
Endosymbiotic Hypothesis of Mitochondria and
Chloroplasts
•
Eukaryotes
display presence
of bacterial
genes encoding
for energy
metabolism
•
Aerobic bacteria
gave rise to
mitochondrion
•
Cyanobacterium
gave rise to
chloroplasts of
green algae,
plants, and red
algae
History of Endosymbiosis
• Mitochondria derived from proteobacterium capable of
aerobic metabolism
• Chloroplasts appear in several distantly related protist
clades
– Photosynthetic pigments differ
– Not all chloroplasts have a pair of membranes
• Some have three
• Primary endosymbiosis
– All chloroplasts trace their ancestry back to engulfment of a
cyanobacterium
• Chlorophyll a present in all!!
• One membrane from cyanobacterium, second from host
– Gave rise to chloroplasts of green and red algae
• Secondary and tertiary endosymbiosis
– All other photosynthetic protist lineages
Eukaryotes Acquired Features From
Both Archaea and Bacteria
Why aren’t the three
rRNA genes of corn one
another’s closest
relatives?
How would you explain
the closer relationship of
the mito rRNA gene of
corn to the rRNA gene of
E.coli than to the nuclear
rRNA genes of other
eukaryotes?
Can you explain the
relationship of the rRNA
gene from the chloroplast
of corn to the rRNA gene
of the cyanobacterium?
If you were to sequence the rRNA genes from
human and yeast mito genomes, where would
you expect these two sequences to fit on the
gene tree?
Origins of Multicellularity
• Major multicellular lineages in eukaryotes: brown algae, plants,
animals, fungi, red algae
• Some protists form colonies or are primitive multicellular
– Examples: Volvox, colonial; some algae, multicellular
– True multicellular organisms - there is a division of labor
among different kinds of cells and non are independent
• Multicellularity
– Increase in size advantageous
• Less predation
• Lower metabolic rate --> less food requirement
Origins of Multicellularity
• What is required for multicelluarity?
– Cohesion
• Animals = extra cellular matrix and collagen
• Plants = plasmodesmata and cell walls
– Communication between cells to allow for cooperation
• Nervous system and endocrine system in animals
• Signal transduction and hormones in plants
– Developmental plan
• Regulation of gene expression to guide specialization of cells
– Cells must specialize!!
• See distinctive organelles become more or less prevalent in
different cell types
– Can’t just increase size b/c SA/V ratio
Origins of Multicellularity
• Cells must specialize!!
– See distinctive organelles become more or less prevalent in
different cell types
– Can’t just increase size b/c SA/V ratio
•
•
•
•
Flexible membrane surface
Change shape
Activity near surface
Vacuoles for food storage/waster removal
– Food vacuoles
– Contractile vacuoles (help eliminate excess water)
• Multicellularity
» Eventually increase cell number
Origins of Sexual Reproduction
• Sexual reproduction allows for adaptive radiation and lost of
new species
– Genetic recombination and exchange
• Protists exhibit both asexual and sexual reproduction
– Asexual: binary fission, budding, spores
– Sexual:
• Conjugation
• Haploid life cycle, alternation of generation life cycle
– Gametes haploid
– Both diploid and haploid cells undergo mitosis
• Diploid life cycle
– Gametes haploid
– Only diploid cells undergo mitosis
• Eukaryotic reproduction requires existence of mitosis and
meiosis!!!!
– Both processes dependent on linear chromosomes and spindle
fibers (microtubules of cytoskeleton)
Figure 27.13 Alternation of Generations
Life Cycles
Plants
• Isogamy vs oogamy
– Gametes look alike; gametes from each gender distinct
• Sporophyte vs gametophyte
– Gives rise to spores; gives rise to gametes
• Isomorphic vs heteromorphic
– Haploid and diploid individuals distinct; haploid and diploid
individuals appear alike
Animals
Haploid Life Cycle:
Chlamydomonas
• Individual is haploid
• Gametes form by
mitosis
• Gametes fuse to
form zygote
– Zygote undergoes
meiosis to form
spores that grow by
mitosis to form new
individual
– NO DIPLOID
INDIVIDUAL!!!!
Figure 27.15 A Haplontic Life Cycle
Alternation of generations:
Ulva and Laminaria
•
Ulva is green algae
–
Isogamic
•
–
Isomorphic
•
–
•
Both generations phenotypically similar
but differ in ploidy
Alternation of generations
Laminaria: brown algae
–
Oogamy
•
–
–
Fertilization of egg by sperm (gametes
phenotypically distinguished, generally
egg predominant)
Heteromorphic
•
•
Phenotypically indistinguishable
gametes
Generations phenotypically distinct
Alternation of generations
Plants
–
–
–
Oogamic (most)
Heteromorphic
Alternation of generations
Figure 27.14 An Isomorphic Life Cycle
Diploid Life Cycle:
Fucus
• Fucus: Brown Algae
• All animals
• Only individual is
diploid
• Gametes by meiosis
Survey of the
Protists
• Cell wall or test
• Absence of cell wall
– Contractile vacuoles for
maintaining osmotic
pressure
• Type of nutrition
– Photoautotroph
– Heterotroph by ingestion
– Heterotroph by absorption
• Locomotion
–
–
–
–
•
Non-motile
Flagella
Cilia
pseudopodia
Note - this arrangement does not follow
phylogeny - grouped in many textbooks
by convenience!
Animal-like Protists
• In general
–
–
–
–
Motile
Unicellular or colonial
Wall-less
Heterotrophic
• Protists with Pseudopodia
• Rhizopodia (amoebas)
– Pseudopodia
– Entamoeba histolytica - amoebic dysentery
• Foraminifera (forams)
– Tests made of CaCO3
– Long-hairlike pseudopodia that poke out thru holes of test
• Actinopoda (radiolarians)
– Silicon test
– Long-hairlike pseudopodia that poke out thru holes of test
Animal-like Protists
• Ciliophora (cilliates)
– Complex organization
– Asexual reproduction; sexual by conjugation involving micronuclei
Animal-like Protists
• Zoomastigophora (zooflagellates)
– Generally symbionts or parasites
– Giardia
– Trypanosoma
• Chagas
• African sleeping sickness
– Choanoflagellates
• Closest relative to Animals!!!!!! (rRNA analysis)
Animal-like Protists
• Apicomplexa
(sporozoans)
– Nonmotile, parasitic,
spore-forming
– Complicated life-cycles
– Toxoplasma
– Plasmodium: malaria
Figure 27.27 A Link to the Animals
Figure 27.1 Major Eukaryote Groups in an Evolutionary Context (Part 1)
Plant-like Protists
•
Pyrrophyta (dinoflagellates)
– 2 flagella; one wraps around middle
of cell
– Cell protected by celluose/silica
plates
– Chlorophylls a and c, carotenoids
– Red-tide
•
Chrysophyta (golden-brown algae;
diatoms)
– Diatoms formally called
Bacillariophyta
• Diatoms have cell wall of silica; major
component of phytoplankton
– Chlorophyll a and c, fucoxathin
•
Euglenophyta (eugleniods)
– 1/3 have chloroplasts, rest do not
– Chloroplasts like those of green
algae
• Chlorophyll a, b and carotenoid
– 2 flagella
– No cell wall
– Eyespot to detect light
Plant-Like Protists
• Chlorophyta (green algae)
– Closest relatives to plants
• Chlorophyll a, b, carotenoids
• Store food as starch
• Walls of cellulose
– Lichens: green algae + fungi
• Rhodophyta (red algae)
– Unicellular to multicellular
– Chlorophyll a, phycobilins
– Food stored as floridian starch
• Phaeophyta (brown algae)
– All multicellular
– Chlorophylls a and c, fucoxanthin
– Store good as laminarin
Figure 27.23 Chlorophytes
Figure 27.22 Red Algae
Figure 27.20 Brown Algae
Importance of Protists
• Link to eukaryotic origins
• Impact on Human Health
– Plasmodium (malaria)
– Trypanosoma (African sleeping sickness; Chagas)
• Ecological Importance as Primary Producers
– Dinoflagellates
• Marine phytoplankton
• Endosymbiotic with corals
• Ride tides and algal blooms
– Diatoms
• Marine phytoplankton
• Common in fresh water
• Diatomaceous earth
– Chlorophytes (green algae)
• Links to origins of animal kingdom - choanoflagellates
Figure 27.8 Dinoflagellate Endosymbionts are Photosynthesizers
Figure 27.10 Chromalveolates Can Bloom in the Oceans
Figure 27.19 Diatom Diversity